Optical fibres are widely used in the communications field because of their ability to carry large amounts of information over long distances. In order to protect the fibres from physical damage during installation and also from subsequent deterioration due to environmental agencies, it is conventional to apply protective coatings to the freshly drawn fibres as an integral part of the production process.
Because of the difficulty of achieving all the required physical properties in a single coating, optical fibres are frequently provided with at least two coatings (normally organic polymer-based) a soft (primary) buffer coating, nearest to (usually contacting) the optical fibre, having an ability to compensate (inter alia) for the effects of differential thermal expansion, and a secondary high modulus coating, providing (inter alia) the necessary toughness and resistance to chemical attack. Such a coated optical fibre may also have one or more further coatings in addition to the primary (buffering) and secondary high modulus coatings to ameliorate still further the properties of the protective coating as a whole; such further coatings may be outer to or inside the secondary high modulus coating. It is also possible that an effective protective coating might be achieved with a single coating, particularly as the technology advances, which then serves as the coating providing toughness and chemical resistance as well as a buffering layer.
Coated optical fibres, in whatever structural variation, are herein termed optical fibre assemblies; such assemblies may, for example, contain a single buffered fibre (normally with at least one further coating), a plurality of fibres encapsulated in a common buffer coating (usually with at least one further coating), or a bundle of bare or coated fibres loosely contained or firmly held within a polymeric sheath.
The hitherto normal method of installation of optical fibre assemblies involves pulling the assembly along previously laid cable ducts with the aid of ropes, and to avoid damage to the assemblies, it is necessary to overjacket them with an expensive cable system.
In order to avoid these problems, it has been proposed in EP-A-0108590 to propel the fibre assembly along a tubular pathway, provided by a duct of a transmission line, by the fluid drag of a pressurised gaseous medium (normally air) passed through the pathway in the desired direction of advance. In other words, the optical fibre assemblies are blown into place on a cushion of air. Using this technique it is proposed to "blow" optical fibres along ducts for long distances without damage. It has also been proposed to withdraw the fibre assemblies from transmission lines by the same kind of technique. A single transmission line can have a single duct therein or can have plurality of such ducts therein, each of which may carry, as has the potential for carrying, an optical fibre assembly.
In order to achieve an effectively blowable optical fibre assembly, various design features therefor have been proposed in the prior art.
For example, in EP-A-157610 an optical fibre assembly is described in which optical fibres are surrounded by an inner sheath and in turn an outer layer of lower density, typically foamed polyethylene, to improve blowability. U.S. Pat. No. 4,952,021 similarly incorporates a foamed outer layer in an optical fibre assembly to improve blowability. EP-A-345968, on the other hand, incorporates particles such as hollow glass or polymeric microspheres in the external coating of an optical fibre assembly in order to achieve reduced density. This publication also teaches incorporating particles such as hollow microspheres or polytetrafluoroethylene (PTFE) particles into the external coating of a size, relative to the coating thickness, such that the surface of the coating is provided with indentations and/or protuberances; the resulting surface roughness of the external coating provides a more effectively blowable system.
In GB-A-2156837 and U.S. Pat. No. 4,952,021 there are described optical fibre assemblies in which the external coating of the optical fibre assembly, and/or the duct through which it passes, carries a friction or adherence-reducing substance such as an antistatic agent, a slip or lubricating material, or an antiblock agent. Combinations of such materials are also contemplated in these teachings.
In our experience, the build-up of a static electric potential between the optical fibre assembly and the duct, caused by the frequent contacting of the fibre assembly and the duct wall as the fibre assembly passes through the duct, is an especially troublesome feature which inhibits effective blowability of the optical fibre assembly, and it is therefore highly desirable that any accumulation of static charge on the fibre assembly (and duct) should be eliminated as soon as possible after its formation. This formation of accumulated static charge is particularly a problem in the case of optical fibre assemblies which carry an external coating based on a radiation-cured urethane-acrylate resin which, while an excellent material for the purpose, does tend to accumulate static charge to a greater extent in comparison to coatings such as foamed polyethylene and moreover tends, unaided, to dissipate this charge more slowly in comparison to certain other known external coating materials such as ethylene/vinyl acetate copolymers. It is nevertheless desirable to dissipate as soon as possible accumulated static charges formed in any type of optical fibre assembly, and for this purpose it is known, as mentioned above, to incorporate an antistatic agent in the external coating of the optical fibre assembly and/or in the duct material (as described e.g. in GB-A-2156837 and U.S. Pat. No. 4,952,021). The antistatic agent in an external coating of an optical fibre assembly is believed to exert its charge dissipating effect by migration (blooming) to the surface of the coating (and similarly for the duct material if it too carries an antistatic agent) and there causing an increase in surface conductivity (and hence more ready dissipation of static charge). (For example, it is considered by many of those skilled in the art that antistatic behaviour corresponds to a surface resistivity of .ltoreq.10.sup.-- ohm/square). We do not, however, wish to be bound by this theory of the mechanism for effecting the dissipation of static charge. Moreover, whatever the precise mechanism, substances which can act to dissipate static charges are extremely well known in the literature and to those skilled in the art and are conventionally called antistatic agents (with the resulting effect being known as antistatic behaviour) a term which we will use herein irrespective of the precise mechanism of charge dissipation.
Antistatic agents, or compositions containing antistatic agents, are well known in the art as mentioned above (many being available commercially) and can be incorporated into optical fibre assemblies (and/or ducts) for the purpose discussed above. Some of these are aqueous dispersions of antistatic agents such as quaternary ammonium compounds. However, there can be difficulties associated with formulating such aqueous dispersions into compositions to be used for optical fibre coating; e.g. they may give rise to globule formation, settlement and general inhomogeneity in the finally formulated composition. In the case of commercially available antistatic agents, or compositions containing them, which do not incorporate water to any (or any substantial) extent, most are only effective if the moisture content of the gas used for blowing (usually air) is quite high. Indeed many water-lacking antistatic agents, or compositions containing them, including those available commercially, are not properly effective unless the relative humidity level of the atmosphere is .gtoreq.30%(at ambient temperature and pressure). However, the presence of such high levels of moisture in the pressurising gas can be undesirable because water can condense and hence collect at the launch end of the duct (because of the high pressure), and also in the cooler regions of the duct, and this can result in difficulties in blowing the fibre through the wet duct. It is also operationally easier not to have to wet the air to achieve high levels of moisture.
It would therefore be most desirable for an antistatic agent, in the context discussed above, which does not incorporate water to any (or any substantial) extent, to be properly effective at low levels of moisture in the gas used for blowing, e.g. at 0 to 8% relative humidity (at ambient temperature and pressure), as well as at higher levels of up to 30% or more relative humidity. In other words, it would be most desirable if the effectiveness of the antistatic agent were substantially independent of the level of relative humidity and could be used without detriment at any moisture level in the pressurised gas, particularly at very low levels of moisture.
In addition to this desiteratum, it would also be advantageous if this moisture-independent antistatic effect did not deteriorate on ageing, even at elevated temperatures, since optical fibres, once installed, will tend to be left for many years.